A display capacity of a liquid crystal display system using a liquid crystal panel has been recently increased.
In a simple matrix structured liquid crystal display system employing a multiplex driving system, a contrast is dropped or a response speed is reduced as a duty ratio is higher. Accordingly, if the liquid crystal display system has about 200 scanning lines, it is difficult to obtain a sufficient contrast.
Accordingly, there has been employed an active matrix system liquid crystal panel having switching elements in each pixel to remove such drawbacks.
There are two types of active matrix system liquid crystal display panel, namely, one is a three terminal devices employing thin film transistors as switching elements and the other is two terminal devices employing nonlinear resistors. The two terminal devices is superior to the three terminal devices since the former is simple in structure and a method of manufacturing thereof.
A diode type, a varistor type, or an MIM (Metal-Insulator-Metal) type is developed as the two terminal devices.
Among these types, the MIM type is simple in structure and has few manufacturing steps.
Further, the liquid crystal panel requires high density and high definition, and the switching elements require reduction of their occupied areas.
As a means for achieving high density and high definition, there are a photo-lithography technique and an etching technique which are respectively micro processing techniques of a semiconductor production technique. However, even if this semiconductor production technique is employed, it is very difficult to realize a large area with low cost.
A structure of a switching element which efficiently makes the area large with low cost will be now described with reference to FIG. 11, showing a plan view exemplifying a conventional liquid crystal display system, and FIG. 12 showing a cross sectional view taken along the line XII--XII of FIG. 11.
The liquid crystal display system comprises, as shown in FIG. 12, a first substrate 131, a second substrate 136 which are respectively made of a transparent material and opposing each other by way of a spacer 142 at a given gap, and a liquid crystal 141 which is filled between the first and second substrates 131 and 136.
A first electrode 132 and a display electrode 135 are respectively arranged in a matrix on the first substrate 131 as shown in FIG. 11, and a nonlinear resistor layer 133 is disposed on the first electrode 132. Further, a second electrode 134 is disposed on the nonlinear resistor layer 133 to overlap the nonlinear resistor layer 133 so as to constitute a nonlinear resistor 130. The second electrode 134 extends from the display electrode 135 and a part thereof serves as a display electrode as shown in FIG. 11.
A black matrix 137 is disposed at an entire region as hatched in FIG. 11 on a surface of the second substrate 136 opposing the first substrate 131 so as to prevent leaking light from the gaps between each display electrode 135 disposed on the first substrate 131. That is, the black matrix 137 is disposed on a non-display electrode portion as a shading portion.
Still further, an opposed electrode 139 is disposed on the second substrate 136 opposing the display electrode 135 by way of an insulating film 138 as shown in FIG. 12, and it is arranged in a belt-shape as shown in FIG. 11 so that the opposed electrode 139 is prevented from contacting the black matrix 137 to be short circuited.
In FIG. 11, the first electrode 132, the display electrode 135, and the second electrode 134 on the first substrate 131 are respectively shown by broken lines, and the nonlinear resistor layer 133 is omitted, and the black matrix 137 and the opposed electrode 139 respectively under the second substrate 136 are shown by solid lines.
The first electrode 132 on the first substrate 131 has an protruding region 132a to provide the nonlinear resistor 130 thereon, and the protruding region 132a overlaps the second electrode 134 to constitute the nonlinear resistor 130.
A given gap d is defined between the first electrode 132 and the display electrode 135.
The display electrode 135 is arranged to overlap the opposed electrode 139 by way of the liquid crystal 141, thereby forming a pixel portion of the liquid crystal panel.
The black matrix 137 is provided to overlap a region forming the display electrode 135 by a given amount so as to prevent leaking light from the peripheral region of the display electrode 135.
The liquid crystal display system carries out a prescribed image display owing to the change of transmittance of the liquid crystal 141 on a region where the black matrix 137 on the display electrode 135 is not formed.
Further, orientational films 140 are provided on opposite surfaces of the first and second substrates 131 and 136 as a processing layer for regularly arranging molecules of the liquid crystal 141.
Meanwhile, there is a nonlinear resistor showing asymmetric characteristic changes due to the polarity of a voltage to be applied thereto. An example of characteristics of the nonlinear resistor having such an asymmetric characteristic will be now described with reference to the drawings.
FIG. 13 is a graph showing a voltage-current characteristic of a nonlinear resistor comprising a tantalum (Ta) film as a first electrode, a tantalum oxide (Ta.sub.2 O.sub.5) film as a nonlinear resistor layer, an indium-tin oxide (ITO) film which is a transparent conductive film as a second electrode.
In this graph, each curve L shows an initial characteristic of the nonlinear resistor. On the other hand each curve M shows a characteristic of the nonlinear resistor after it is driven.
When a positive (+) voltage is applied to the first electrode of the nonlinear resistor, a value of the current which can flow through the nonlinear resistor with the same voltage is largely reduced as shown by the curve M representing the characteristic of the nonlinear resistor after it is driven compared with the curve L representing the initial characteristic of the nonlinear resistor.
When a negative voltage is applied to the first electrode of the nonlinear resistor, a value of the current which can flow through the nonlinear resistor with the same voltage is scarcely reduced as shown by the curve M representing the characteristic of the nonlinear resistor after it is driven compared with the curve L representing the initial characteristic of the nonlinear resistor.
Denoted by P is a difference between the curve L presenting the initial characteristic and the curve M representing the characteristic after the nonlinear resistor is driven when a positive voltage is applied to the tantalum film serving as the first electrode. Likewise, denoted by Q is a difference between the curve L presenting the initial characteristic and the curve M representing the characteristic after the nonlinear resistor is driven when a negative voltage is applied to the first electrode.
The difference P when the positive voltage is applied to the first electrode is greater than the difference Q when the negative voltage is applied to the first electrode as evident from FIG. 13.
FIG. 14 is a graph showing changes of the differences P and Q with respect to a driving time. A curve R shows the change of the difference P with respect to the driving time when the positive voltage is applied to the first electrode, wherein a current value is shapely increased as a driving time elapses.
On the other hand, a curve S shows the change of the difference Q with respect to the driving time when the negative voltage is applied to the first electrode, wherein a current value is scarcely changed even if the driving time elapses.
This is shown by a difference U between the curves R and S, wherein the difference U is shapely increased as the driving time elapses.
The difference U is changed depending on the driving time, the amount of current flowing through the nonlinear resistor, an environment for driving the nonlinear resistor or a history of the nonlinear resistor.
Accordingly, it is very difficult to compensate the change of the difference U.
A voltage to be applied to each liquid crystal pixel is differentiated between a case where the positive voltage is applied to the first electrode 132 of the nonlinear resistor 130 and a case where the negative voltage is applied to the first electrode 132 of the nonlinear resistor 130 owing to the occurrence of the difference U.
As a result, drop of contrast, image flicker, and image sticking because of after-images owing to deviation of ions in the liquid crystal occur, thereby causing a problem of significant deterioration of a display quality of the liquid crystal display system.
It is an object of the present invention to solve such problems. That is, it is an object of the present invention to suppress asymmetric characteristic changes due to the difference of polarity of the voltage to be applied to the nonlinear resistor, thereby reducing a DC voltage to be applied to the liquid crystal, eliminating deviation of ions in the liquid crystal, preventing the drop of contrast, flicker and image sticking, thereby enhancing the image display quality of the liquid crystal display system.